Subtle bug/difference difference in Python 2 and 3 with SavWriter.spssDateTime

Hi,

and thanks for an excellent library.

When porting my program from Python 2 to 3, I discovered a subtle difference when calling SavWriter.spssDateTime:

#python 2:
writer.spssDateTime("2000-01-01", "%Y-%m-%d") ## this will return 13166064000.0

#python 3:
writer.spssDateTime("2000-01-01", "%Y-%m-%d") ## this will return sysmis (aka -1.7976931348623157e+308)

While I now recognize that the first argument to spssDateTime should be binary, Python 2 is happy with a string. spssDateTime in Python 3 on the other hand will throw an exception on datetimeStr.decode("utf-8"), catch the exception and return sysmis.

I therefore propose the following to support both strings and binary in Python 2 and 3:

    def spssDateTime(self, datetimeStr=b"2001-12-08", strptimeFmt="%Y-%m-%d"):
        """ This function converts a date/time string into an SPSS date,
        using a strptime format. See also :ref:`dateformats`"""
        try:
            if type(datetimeStr)==bytes:
                datetimeStr = datetimeStr.decode("utf-8")
            dt = time.strptime(datetimeStr, strptimeFmt)
        except (ValueError, TypeError, AttributeError):
            return self.sysmis
        day, month, year = dt.tm_mday, dt.tm_mon, dt.tm_year
        hour, minute, second = dt.tm_hour, dt.tm_min, dt.tm_sec
        return (self.convertDate(day, month, year) +
                self.convertTime(0, hour, minute, second))

Feedback? Should I make a pull request? It's just a single new line of code and an indentation of the following line.

Best regards Ivar Refsdal

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